1. Field of the Invention
This invention relates to a control system for internal combustion engines, and more particularly to a control system of this kind, which controls the air-fuel ratio of an air-fuel mixture supplied to the engine in a feedback manner, by applying an adaptive control theory thereto or by the use of a controller of a recurrence formula type.
2. Prior Art
Conventionally, an air-fuel ratio control system for internal combustion engines has been proposed e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 3-185244, in which an optimal regulator which is one of modern control theories is applied to air-fuel ratio feedback control based on an output from a linear-output oxygen concentration sensor (LAF sensor) arranged in the exhaust system of the engine, whereby the air-fuel ratio feedback control is carried out by the use of the output from the LAF sensor and an optimum feedback gain calculated based on a dynamic model representative of a behavior of the engine (adaptive control).
In the air-fuel ratio feedback control having the optimal regulator applied thereto, a recurrence formula representing a correlation between a result of measurement or detection of the air-fuel ratio and control amounts applied in the past is prepared, to thereby obtain an optimal value of the control amount to be applied in the following execution of the air-fuel ratio control. The air-fuel ratio feedback control utilizing the optimal regulator is high in control response, and is therefore very effective means for controlling the air-fuel ratio of the mixture so long as the engine combustion continues to be in a steady or stable condition.
However, if the air-fuel ratio feedback control utilizing the optimal regular is carried out when the engine is decelerating, the engine combustion becomes unstable, which results in large variations in the detected air-fuel ratio of the mixture. To suppress such undesired variations of the air-fuel ratio, there has been proposed an air-fuel ratio control system for an internal combustion engine by Japanese Laid-Open Patent Publication (Kokai) No. 4-209940, which, when the engine is decelerating, applies an optimal feedback gain which imparts lower responsiveness to the control system or effects changeover from adaptive control to proportional term/integral term control (PI control).
However, the air-fuel ratio feedback control utilizing the optimal regulator relies on results of control operations executed in the past. Therefore, when the engine is in an operating condition where continuity of the control cannot be maintained, erroneous or unsuitable control amounts can be calculated even when the engine is not decelerating. For example, if the engine combustion is not in a steady or stable state, hunting can occur due to the high responsiveness of the feedback control utilizing the optimal regulator. That is, the accuracy and stability of the control by the use of the optimal regulator lowers when the air-fuel ratio of the mixture undergoes a drastic change, the engine combustion is degraded, the air-fuel ratio of the mixture cannot be detected with accuracy, or a correct amount of fuel corresponding to a calculated control amount is not supplied to the engine, for example. Therefore, from the viewpoint of driveability and exhaust emission characteristics of the engine, the proposed air-fuel ratio feedback control utilizing the optimal regulator has room for improvement. This problem becomes very serious, especially with an adaptive controller using a recurrence formula, which has still higher responsiveness than the optimal regulator.
On the other hand, an air-fuel ratio control system has been proposed by Japanese Patent Publication (Kokai) No. 60-56353, which interrupts execution of air-fuel ratio feedback control for controlling the air-fuel ratio based on an output from an oxygen concentration sensor, which detects the concentration of oxygen in exhaust gases emitted from the engine, when the oxygen concentration sensor is determined to be abnormal.
Further, there has been also proposed an air-fuel ratio control system by Japanese Laid-Open Patent Publication (Kokai) No. 5-52140, which interrupts execution of the feedback control (adaptive control) based on an optimal feedback gain calculated by the use of the optimal regulator described above when the linear-output air-fuel ratio sensor is in a half-warmed-up condition (i.e. before it is fully warmed up), but executes the air-fuel ratio feedback control by the PID method.
However, the air-fuel ratio control system disclosed in Japanese Patent Publication (Kokai) No. 60-56253 interrupts execution of the feedback control when abnormality of the oxygen concentration sensor has been detected, and therefore there is room for improvement in respect of exhaust emission characteristics and driveability of the engine during the interruption of the feedback control.
The air-fuel ratio control system disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 5-52140 contemplates a half-warmed-up condition of the linear-output air-fuel ratio sensor, but does not contemplate abnormality of sensors including the linear-output air-fuel ratio sensor. Therefore, there remains room for improvement in respect of exhaust emission characteristics and driveability of the engine which would be degraded when any of the sensors is abnormal.
Further, an air-fuel ratio control system is known, which carries out PID air-fuel ratio control of a mixture supplied to an internal combustion engine, based on an output from a linear-output air-fuel ratio sensor arranged in the exhaust passage by the use of a proportional term, an integral term, and a differential term, and detects abnormality of a fuel supply system of the engine by the use of the integral term.
However, if this abnormality-detecting method is directly applied to an air-fuel ratio control system using the aforementioned optimal regulator having such fairly high control speed that correction coefficients calculated during the feedback control are changed at high speeds and by fairly large amounts, it is impossible to detect abnormality of the fuel supply system with accuracy.